As a mobile communication system, 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) mobile communication system has been known. FIG. 9 shows an example of a general system configuration of the known LTE mobile communication system. Referring to FIG. 9, a mobile terminal 101 is in a cell (not shown) that is the service area of a base station 100.
As shown in FIG. 10, upon calling (at the beginning of random access communication), the mobile terminal 101 sends a PRACH (Physical Random Access Channel) preamble, which notifies a desire to start the random access communication, to the base station 100 by uplink 102 prior to sending a message. The mobile terminal 101 selects one from various kinds of preamble patterns and generates a preamble according to the selected preamble pattern.
The base station 100 receives the preamble, and detects a pattern of signals of the received preamble to obtain a correlation between the received preamble and a predetermined preamble pattern. Then, the base station 100 determines whether the base station 100 should send a message in response to the detected preamble pattern. When the base station 100 approves the message sending, the base station 100 sends an ACK (Acknowledge) to the mobile terminal 101 through a PDSCH (Physical Downlink Shared Channel). When the base station 100 disapproves the message sending, the base station 100 sends nothing. When the base station 100 has searched all kinds of the preamble patterns and has detected nothing that correlates with the received preamble pattern, the base station 100 sends nothing, either.
When the mobile terminal 101 receives the ACK in a certain period after sending the preamble, the mobile terminal 101 sends the message to the base station 100 through the uplink 102. If the mobile terminal 101 detects that no response has been received from the base station 100, the mobile terminal 101 repeats the above-described operation.
FIG. 11 shows an example of a preamble pattern detection circuit at a base station in such a mobile communication system. Referring to FIG. 11, the preamble pattern detection circuit has a frequency converter 1, an FIR (Finite Impulse Response) digital filter 2, a 2048-point FFT (Fast Fourier Transformer) 3, a correlation detector 4, and a peak detector 5.
The frequency converter 1 is for performing frequency shift on signals sent from the mobile terminal by uplink as input signals. The FIR filter 2 is for limiting the bandwidth and lowering the sampling frequency by performing low pass filtering on the frequency converted output. The 2048-point FFT 3 is for transforming a time domain signal into a frequency domain signal by performing 2048-point FFT processing on the bandwidth limited signal.
The correlation detector 4 is for obtaining cross-correlation values between the frequency domain signal and a plurality of predetermined preamble patterns and transforming the results into the time domain signal. For that purpose, the correlation detector 4 has a multiplier 41 for multiplying the frequency domain input signal and each of a plurality of the predetermined preamble patterns; and a 2048-point IFFT (Inverse FFT) 42 for transforming each multiplied output into the time domain signal.
The peak detector 5 is for detecting a peak of cross-correlation values output from the correlation detector 4, and deriving a preamble pattern number corresponding to the detected peak and a peak detected timing as preamble detection output. Japanese Patent Laid-Open No. 2007-259326 discloses an example of that kind of circuit.
Four uplink signals are defined for the uplink signals in the LTE mobile communication system: PUSCH (Physical Uplink Shared Channel), PUCCH (Physical Uplink Control Channel), Sounding RS (Reference Signal), and the above-described PRACH. These signals are assigned to different resource element, respectively. The resource element is an information element uniquely defined by two indices of an index of a narrowband carrier called subcarrier and an index of a time symbol.
The base station is adapted to transform the signal received from the mobile terminal by the uplink into the frequency domain signal by performing FFT processing on the signal and to detect data patterns (the above-described four kinds of signal information) for each subcarrier in the frequency domain.
FIG. 12 shows a signal format of PRACH preamble by one sub-frame. FIG. 13 shows an example of a signal format of an uplink signal excluding the PRACH preamble by one sub-frame. The PRACH preamble is composed of a portion called CP (Cyclic Prefix); a preamble portion including 24576 samples; and a portion called Guard Interval. The uplink signal excluding the preamble signal is composed of #0 to #6 symbols, each of which is composed of the CP and 2048 sample data.
Accordingly, the number of points for the FFT processing performed in the base station is usually 2048 samples. Thus, the preamble pattern detection circuit shown in FIG. 11 also uses 2048-point FFT 3. Since the above-described PRACH preamble that is sent at the random access has a symbol length of 24576 samples, which is longer than that of the other uplink 2048 sample signal, the processing by the 2048-point FFT 3 is repeated by 24576/2048=12 times. That increases the number of points for the FFT processing in the preamble pattern detection circuit, and also increases the throughput.
Reducing the number of points for the FFT can be a solution for reducing the throughput. A method is disposing the FIR filter 2 prior to the FFT 3 at the first stage for the purpose of performing the low pass filtering on the signal to limit the bandwidth and lower the sampling frequency before performing the FFT processing. The method is a down sampling approach in the time domain.
The method, however, requires a steep frequency characteristic for the FIR filter for correctly extracting the subcarrier of the preamble. In order to realize the steep frequency characteristic, the FIR filter needs to have more taps, which disadvantageously results in much more computational complexity in the FIR filter processing.
The above description can be summarized as below: The LTE mobile communication system is adapted to have the frequency bandwidth of the system divided into the narrowband subcarriers and transmit the respective signals. Since the pattern detection processing is performed on the received signal at the base station in the frequency domain, the processing is based on the FFT and IFFT processing with much computational complexity. In addition, as the number of the mobile terminals to be processed increases, the pattern detection processing performed at the base station also increases. That disadvantageously leads a processing delay and slows down the communication speed.